Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Beyond Compare - Beyond Compare allows you to compare files and folders.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
WinMerge - WinMerge is an open source differencing and merging tool for Windows.
Invision - Prototyping and collaboration for design teams
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.